Father misunderstands that and just sees it as true power. He sees being god as being a being without flaws, which is why he creates his homunculi out of his sins....but he's not actually separating himself from them, is he? Even though Greed, Wrath, Pride, and Sloth are outside his body, he still wants overwhelming power for himself, he tries to destroy anyone who is against him, he considers himself to be high above everyone else, and he sits on his ass while his children do all the work. If anything, he's more sinful without them. When Greed was temporarily back in his body, he generously healed Ed and Al (though it did serve his purposes to some extent, he hardly needed to do it).
My interpretation of the character is that him being in the human world has driven him insane. He's an entity from beyond the gate and he doesn't belong in the human world, so he's trying to go back in a subconscious way (he wants to bring 'god' and thus whats beyond the gate of truth to himself), but in his insanity, he's forgotten what 'god' actually is. The same way he thinks he can be 'sinless' while still committing atrocities just because he put his sins in his homunculi, he thinks taking the power of God will let him be God himself.
